Braid-reinforced hollow fiber membrane

ABSTRACT

A braid-reinforced hollow fiber membrane which includes a reinforcing material of a tubular braid and a polymer resinous thin film coated on the surface of the reinforcing material. The polymer resinous thin film has a skin layer with micro pores having a diameter in the range from 0.01 to 1 μm and an inner layer of a sponge structure with micro pores having a diameter less than 10 μm. 
     The hollow fiber membrane exhibits excellent mechanical strength, reliable filtration and simultaneously, good water permeability.

This nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Application No. 2001-0077181 filed in KOREA on Dec. 7, 2001,which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymer separation membrane having anexcellent chemical resistance, filtration reliability, mechanicalstrength and water permeability, and more particularly, to a compositehollow fiber membrane.

Recently, polymer separation membranes are being utilized in morevarious fields as well as existing application fields with theimprovement of their techniques. Particularly, with the importance ofenvironment, demands for them are being increased in the fields of watertreatment. In all application fields of separation membranes, amechanical strength always stands out as an important factor as well asselectivity and water permeability. Particularly, in water treatmentfields, an excellent mechanical strength is necessarily required,simultaneously with a high permeability, from the viewpoint of thereliability of a separation membrane system.

A hollow fiber-shaped membrane has a high permeability per installationarea and is suitable for water treatment, whereas the mechanicalstrength thereof has been a problem to be solved due to thecharacteristics of a porous membrane structure. Thus, a hollow fibermembrane is reinforced with a fabric or tubular braid having anexcellent strength as a support of the separation membrane.

2. Description of Related Art

Such a general idea of a composite membrane is a well known fact.Techniques thereof are disclosed in U.S. Pat. No. 4,061,821, U.S. Pat.No. 3,644,139, U.S. Pat. No. 5,472,607 and the like.

Among them, a general idea of a composite hollow fiber membrane using atubular braid was disclosed for the first time in U.S. Pat. No.4,061,821 to Hayano et al. In this technique, however, the tubular braidis not used as a support for coating, but it is completely embedded inthe membrane in order to compensate for a reduction of waterpermeability due to the shrinkage occurred when an acrylonitrile hollowfiber type membrane is solely used at a temperature higher than 80° C.Such a composite membrane has a larger thickness than the thin filmcoated on a support, and the embedded braid increases the resistance offluid flow for thereby significantly reducing the water permeability.

Unlike the prior art, in U.S. Pat. No. 5,472,607, a reinforcing materialis not embedded in the membrane, but is coated on its surface with athin film by coating method of the existing flat membrane. Inmanufacturing a composite hollow fiber membrane having a thin film layercoated on the surface of a reinforcing material or supporting materialof a tubular braid, thermodynamic stability differs according to thecomposition of a dope to be used for coating. This determines thestructure of the coated thin film layer.

That is to say, in case of a thermodynamically stable dope, it has afinger type structure. On the contrary, a dope with a low thermodynamicstability has a sponge structure with no defect region. For instance, inthe case of a dope using a solvent having a strong solvent power such asN-methyl-2-pyrrolidone (NMP) among organic solvents, it can easily forma finger-type structure because it has a high thermodynamic stability.

Additionally, the water permeability and mechanical strength of theoverall composite hollow fiber membrane depends upon the structure andproperties of the thin film layer. This is because the thin film layerhas small pores and a low mechanical strength than a tubular braidreinforcing material having relatively much larger pores and a higherstrength. In other words, the filtrate having passed through the thinfilm layer passes through a braid supporting layer with relatively largepores without a large resistance. While, since the thin film layer has alarge flow resistance, the water permeability of the overall membrane isdetermined according to a microporous structure and porosity of the thinfilm layer.

In view of strength, the tensile strength, pressure resistance and thelike are complemented by the braid reinforcing material having a farsuperior mechanical strength. However, if the strength of the thin filmis reduced, the thin film is separated or damaged.

In U.S. Pat. No. 4,061,821 and U.S. Pat. No. 5,472,607, the significanceof the coated thin film layer structure was overlooked in relation tothe present invention. Particularly, the structure of the thin filmlayer in the two U.S. patents has a porous region larger than 5 μm in aninner layer of a skin, that is, the inner layer has some micro poreshaving a pore diameter higher than 5 μm.

FIG. 2 is an exploded sectional view of a composite hollow fibermembrane disclosed in U.S. Pat. No. 4,061,821; and FIG. 3 is an explodedsectional view of a composite hollow fiber membrane disclosed in U.S.Pat. No. 5,472,607. These membranes are in a finger like structure asshown in FIGS. 2 and 3 and have a macrovoid functioning as a defect inthe thin film layer as seen from the well-known fact.

Thus they acts as a defect in expressing the mechanical properties ofthe thin film. Particularly, when the skin of a dense layer is damaged,a material capable of being cut off by the inner layer is permeated.This reduces the relative filtration reliability of the membrane.

The composite hollow fiber membrane is suitable, particularly forfiltration modules in the fields of water treatment due to its superiormechanical strength. In such a filtration module, there is a possibilityof damaging the surface of the membrane by the friction and physicalimpact generated between membranes due to aeration.

It is an object of the present invention to provide a hollow fibermembrane having an excellent mechanical strength, filtrationreliability, and water permeability by coating a polymeric resinous thinfilm on a braid support.

The present invention provides a hollow fiber membrane having apolymeric resinous thin film, including a skin layer of a densestructure and an inner layer of a sponge structure in which thediameters of the pores are continuously and gradually increased towardto the central axis of the hollow fiber, coated on the reinforcingmaterial of a tubular braid.

In addition, the present invention provides a hollow fiber membranewhich has a high porosity, good mechanical strength and filtrationreliability as well as excellent water permeability by forminggradient-type micro pores smaller than 10 μm in the inner layer of asponge structure of the hollow fiber membrane.

The polymer resinous thin film structure of the hollow fiber membrane ofthe present invention can be made by specifying the composition(including additives) of a spinning dope and regulating thethermodynamic stability of the spinning dope.

SUMMARY OF THE INVENTION

A hollow fiber membrane having excellent mechanical strength, filtrationreliability and water permeability, according to the present invention,includes a reinforcing material made of a tubular braid and a polymerresinous thin film coated on the surface of the reinforcing material,said polymer resinous thin film having a skin layer with micro poreshaving a diameter in the range from 0.01 to 1 μm and an inner layer of asponge structure with micro pores having a diameter less than 10 μm.

The present invention will now be described in detail.

The hollow fiber membrane of the present invention has a structure inwhich a polymer resinous thin film is coated on the surface of thereinforcing material of a tubular braid. The polymer resinous thin filmincludes a skin layer of a dense structure and an inner layer of asponge structure. The skin layer is formed with micro pores having adiameter in the range from 0.01 to 1 μm. The inner layer is formed withmicro pores having a diameter less than 10 μm, preferably, 5 μm.

The present invention contains no porous region larger than 10 μm(hereinafter we call it as “defect region”) in the inner layer of thepolymer resinous thin film, that is, there exist no micro pores having adiameter larger than 10 μm. In the case where any defect region largerthan 10 μm exists in the inner layer, the filtration reliability can begreatly reduced. Preferably, the diameters of micro pores formed in theinner layer of the sponge structure are continuously and graduallyincreased toward to the central direction of the hollow fiber membrane.

To improve both mechanical strength and water permeability, it ispreferable that the thickness of the polymer resinous thin film is lessthan 0.2 mm and the amount of the polymer resinous thin film penetrationinto the reinforcing material is less than 30% of the thickness of thereinforcing material.

The polymer resinous thin film is made from a spinning dope containing apolymer resin, an organic solvent, polyvinylpyrollidone and ahydrophilic compound.

The hollow fiber membrane of the present invention can be made bypassing a tubular braid (reinforcing material) through the centerportion of a double tubular nozzle and simultaneously feeding a spinningdope for the polymer resinous thin film on the surface of the braidthrough the nozzle, coating the spinning dope on the braid, extrudingthem in the air of outside the nozzle, coagulating them in a externalcoagulating liquid to form the hollow fiber membrane, and washing anddrying it.

At this time, the spinning dope for the polymeric resinous thin film isobtained by dissolving a polymer resin, polyvinylpyrrolidone and ahydrophilic compound in an organic solvent. The polyvinylpyrrolidone andhydrophilic compound is are used as an additive. More preferably, thespinning dope is made up of a polymer resin of 10 to 50% by weight,polyvinylpyrrolidone and a hydrophilic compound of 9 to 30% by weightand an organic solvent of 20 to 89% by weight. However, in the presentinvention, the composition ratio of the spinning dope is notspecifically limited.

The polymer resin is a polysulfone resin, a polyethersulfone resin, asulfonated polysulfone resin, polyvinylidenefluoride (PVDF) resin,polyacrylonitrile (PAN) resin, a polyimide resin, a polyamideimde resin,polyesterimide resin and so on. The organic solvent is dimethylacetamide, dimethyl formamide or a solution thereof.

The hydrophilic compound is water or a glycol compound, more preferably,polyethylene glycol having a molecular weight of less than 2,000. Sincethe water or glycol compound, which is hydrophilic, reduces thestability of the spinning dope, it is relatively more likely to form asponge structure.

That is, as the stability of the spinning dope becomes higher, it ismore likely to form a finger-like structure because a defect region(micro pores having a diameter higher than 10 μm) is formed in themembrane. The present invention reduces the stability of the spinningdope by simultaneously adding a water or a glycol compound, and anadditive, to increase the water permeability by making the membranehydrophilic.

Meanwhile, in the process of producing the hollow fiber membrane, inorder to uniformly coat a polymer resinous thin film on the surface ofthe reinforcing material of the tubular braid at a predeterminedthickness, the speed with which the tubular braid is advanced and thequantity of the spinning dope introduced into the nozzle must bebalanced with each other. The relation between the feed rate of aspinning dope and the speed of a tubular braid is expressed by theformula:Q=πρυD_(o) T

[wherein Q denotes the feed rate of dope per time, ρ denotes the densityof dope, υ denotes the advancing speed of the braid, D_(o) denotes theouter diameter of the braid and T denotes the thickness of the dope tobe coated.]

As seen from the above formula, in the case where the advancing speed ofthe braid is high, a thin coating layer is formed. In the case where theadvancing speed of the braid is extremely high relative to the feed rateof the spinning dope, a non-uniform membrane with no coating layer onsome parts is produced. Otherwise, a non-uniform membrane with apartially thick coating layer is produced. Thus there exists an optimumspeed ratio for stably producing a membrane with a uniform thickness.

Preferably, when the ratio (k) of spinning dope feed rate (Q) and braidadvancing speed (v) with respect to a unit outer diameter (D_(o)) ofbraid satisfies the range from 200 to 3,000(g/m²), an optimum coating isachieved. The larger the value of k becomes, the thicker the coatinglayer becomes.

${k\left( {g\text{/}m^{2}} \right)} = \frac{Q\left( {g\text{/}\min} \right)}{{v\left( {m\text{/}\min} \right)}{D_{0}(m)}}$

The performance of the hollow fiber membrane produced when the k valuesatisfies the range from 500 to 2,500(g/m²) is more preferable.

In the polymer resinous thin film layer of the hollow fiber membraneproduced by the above-described method, a dense skin layer and an innerlayer having a sponge structure are formed. The sponge structure of theinner layer becomes gradually larger as it approaches the center of thehollow fiber membrane and has no defect region greater than 10 μm. Thiscan be observed in FIG. 1 showing an scanning electron microscopicphotograph of the cross-section of the membrane by breaking out thepolymer resinous thin film layer of the hollow fiber membrane producedaccording to the present invention.

As described above, since the hollow fiber membrane of the presentinvention is reinforced with a tubular braid and has no defect regiongreater than 10 μm in the inner layer of the polymer resinous thin filmlayer, it has an excellent water permeability, filtration reliabilityand mechanical strength.

In the present invention, the performances and structures of the hollowfiber membrane are evaluated by the following method.

Water Permeability

The water permeability was measured by preparing a mini-module having aneffective length of 25 to 30 cm in the hollow fiber membrane and passingpure water through the module for a predetermined time by out-in flowmethod under a suction pressure of 100 mmHg at a temperature of 25° C.

Solute Rejectivity (Filtration Reliability)

The solute rejection rate was obtained by the following formula bydissolving hemocyanin (with a molecular weight of 2 millions) in abuffer solution, filtering it through a mini-module having an effectivelength of 25 to 30 cm in the hollow fiber membrane by cross flow methodand measuring the concentration of permeated solution from the membraneand the concentration of original (unfiltered) solution by means of UV.

$\text{Solute rejection rate(\%)} = {\frac{\text{concentration of original solution} - \text{concentration of permeated solution}}{\text{Concentration of original solution}} \times 100}$

Mechanical Strength

The tensile strength, tensile elongation and the like of the hollowfiber membrane were measured by a tensile tester. A tensile test wascarried out under a room temperature at a crosshead speed of 3 cm/min ata distance between chucks of 10 cm.

Shape of Micro Pores

The fractured cross-section of the polymer resinous thin film layercoated on the surface of the support (reinforcing material) was observedwith an scanning electron microscope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an scanning electron microscopic photograph showing thecross-section of a polymer resinous thin film of a hollow fiber membraneaccording to the present invention; and

FIGS. 2 and 3 are exploded sectional views of a conventional hollowfiber membrane.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is now understood more concretely by comparisonbetween examples of the present invention and comparative examples.However, the present invention is not limited to such examples.

EXAMPLE 1

A spinning dope is prepared from components: 17% by weight ofpolysulfone, 9% by weight of polyvinylpyrrolidone, and 10% by weight ofpolyethyleneglycol added to 64% by weight of dimethylformamide (organicsolvent), to produce a transparent spinning dope by mixing anddissolving the components. The spinning dope is fed to a biannularnozzle having a 2.38 mmφ diameter and simultaneously a tubular braidhaving an outer diameter of 2 mm is passed through the center portion ofthe nozzle, to thus coat the spinning dope on the surface of the annularbraid and then extrude it in the air. At this time, the ratio (k) of theadvancing speed of the braid to the feed rate of the spinning dope is750 g/m², and the coating thickness of the spinning dope is 0.2 mm.After passing through the tubular braid coated with the spinning dopeinto a 10 cm air gap, it is coagulated in an external coagulating bathwith a temperature of 35° C. The hollow fiber membrane is prepared bywashing in a washing tank and winding. The result of evaluation for thestructure and performances of the produced hollow fiber membrane isshown at Table 1.

EXAMPLE 2

A hollow fiber membrane is produced in the same process and condition asExample 1, except that the spinning dope is composed of 13% by weight ofpolysulfone, 10% by weight of polyvinylpyrrolidone, and 11% by weight ofpolyethyleneglycol and 66% by weight of dimethylformamide. The result ofevaluation for the structure and performances of the produced hollowfiber membrane is shown at Table 1.

COMPARATIVE EXAMPLE 1

A hollow fiber membrane is produced in the same process and condition asExample 1, except that the spinning dope is composed of 17% by weight ofpolysulfone and 19% by weight of polyvinylpyrrolidone and 64% by weightof dimethylformamide (without adding polyethyleneglycol). The result ofevaluation for the structure and performances of the produced hollowfiber membrane is shown at Table 1.

TABLE 1 Structure and Physical Properties of Hollow Fiber MembraneComparative Classification Example 1 Example 2 Example 1 Physical Water150 250 60 Properties Permeability(LMH) Hemocyanin R₀ 89 83 91 RejectionR₁₅ 91 82 87 Rate (%) R₃₀ 89 84 78 Tensile Strength 37 35 31(g_(f)/fiber) Tensile elongation (%) 65 60 57 Structure Thickness(mm) of0.1 0.1 0.15 Coated Thin Film Defect Region larger none none exists than12 μm

In Table 1, R₀ denotes an initial hemocyanin rejection rate, R₁₅ denotesa hemocyanin rejection rate after 15 days under the condition ofaeration with an air flow of 10 L/min, and R₃₀ denotes a hemocyaninrejection rate after 30 days under the condition of aeration with an airflow of 10 L/min As shown in Table 1, in Example 1 and Example 2, sinceR₀, R₁₅ and R₃₀ show values similar to each other, it can be known thatthere is no reduction in hemocyanin rejection rate in a case that thesurface of the membrane is damaged. In other words, Example 1 andExample 2 show an excellent filtration reliability.

However, in Comparative Example 1, R₁₅ and R₃₀ show a value much lowerthan R₀. In other words, in Comparative Example 1 show a very poorfiltration reliability.

INDUSTRIAL APPLICABILITY

The hollow fiber membrane of the present invention is reinforced with asupport of a braid and has no defect region greater than 10 μm in theinner layer (sponge structure) of the polymer resinous thin film.Therefore, the water permeability, mechanical strength and filtrationreliability thereof are excellent. As the result, the hollow fibermembrane of the present invention is particularly suitable forfiltration modules in the fields of water treatment of a large size.

1. A braid-reinforced hollow fiber membrane comprising a reinforcingmaterial of a tubular braid and a polymer resinous thin film coated onthe surface of the reinforcing material, said polymer resinous thin filmconsisting of two layers, a single skin layer with micro pores having adiameter in the range from 0.01 to 1 μm and a single inner layer of asponge structure with micro pores having a diameter less than 10 μm. 2.The braid-reinforced hollow fiber membrane of claim 1, wherein thediameters of micro pores formed in the inner layer of the spongestructure is less than 5 μm.
 3. The braid-reinforced hollow fibermembrane of claim 1, wherein the diameters of micro pores formed in theinner layer of the sponge structure are continuously and graduallyincreased toward the central direction of the hollow fiber membrane. 4.The braid-reinforced hollow fiber membrane of claim 1, wherein thethickness of the polymer resinous thin film is less than 0.2 mm.
 5. Thebraid-reinforced hollow fiber membrane of claim 1, wherein the length ofthe polymer resinous thin film penetration into the thickness of thereinforcing material is less than 30% of the reinforcing material. 6.The braid-reinforced hollow fiber membrane of claim 1, wherein thepolymer resinous thin film is made from a spinning dope containing apolymer resin, an organic solvent, a polyvinylpyrollidone andhydrophilic compound.
 7. The braid-reinforced hollow fiber membrane ofclaim 6, wherein the polymer resin is selected from the group consistingof a polysulfone resin, a polyethersulfone resin, a sulfonatedpolysulfone resin, polyvinylidenedifluoride (PVDF) resin,polyacrylonitrile (PAN) resin, a polyimide resin, polyamideimde resinand a polyesterimide resin.
 8. The braid-reinforced hollow fibermembrane of claim 6, wherein the hydrophilic compound is water or aglycol compound as a spinning dope stabilizer.
 9. The braid-reinforcedhollow fiber membrane of claim 8, wherein the glycol compound ispolyethylene glycol having a molecular weight of less than 2,000. 10.The braid-reinforced hollow fiber membrane of claim 6, wherein theorganic solvent includes dimethyl acetamide, dimethyl formamide or asolution thereof.
 11. A braid-reinforced hollow fiber membrane, whichcomprises a reinforcing material of a tubular braid and a polymerresinous thin film coated on the surface of the reinforcing material,said polymer resinous thin film consisting of two layers, a surface ofthe reinforcing material, said polymer resinous thin film consisting oftwo layers, a single skin layer with micro pores having a diameter inthe range from 0.01 to 1 μm and a single inner layer of a spongestructure with micro pores having a diameter of less than 5 μm, saidhollow fiber membrane being produced by a method in which the ratio (k)of spinning dope feed rate (Q) and braid advancing speed (v) withrespect to the a unit outer diameter (D₀) of braid, which is expressedby the following formula, satisfies the range of from 200 to3,000(g/m²).${k\left( {g\text{/}m^{2}} \right)} = \frac{Q\left( {g\text{/}\min} \right)}{{v\left( {m\text{/}\min} \right)}{D_{0}(m)}}$12. The braid-reinforced hollow fiber membrane of claim 11, wherein theratio (k) of spinning dope feed rate (Q) and braid advancing speed (v)with respect to the a unit outer diameter (D_(o)) of braid satisfies therange from 500 to 2,500(g/m²).